The present invention relates to a method for producing high-purity calcium (Ca) by sublimation purification.
Calcium (Ca) is an alkaline earth metal having an atomic number of 20 and an atomic weight of 40.08 and is broadly distributed as a silicate, carbonate, sulfate, fluoride, or phosphate. Calcium can be purified up to purity of 99.9% by vacuum distillation of calcium having a purity of 94% to 98% prepared by molten salt electrolysis of a material substantially composed of calcium chloride. Purified calcium is a silvery-white soft metal and has a cubic close-packed structure at ordinary temperature, a hexagonal close-packed structure at 250° C. or more, and a body-centered cubic structure at 450° C. or more. Calcium has a melting point of 839° C., a boiling point of 1480° C., and a density of 1.55 g/cm3 (20° C.) and directly binds to oxygen and halogens at ordinary temperature and also reacts with hydrogen, carbon, and nitrogen at high temperature. Calcium is used as a reducing agent, a deoxidizing agent for metals, or a high vacuum getter (see Physical and Chemical Dictionary).
Lanthanum has been researched and developed as an electronic material, e.g., as a metal gate material or a high dielectric constant material (High-k), in recent years and is a metal that is being paid attention. Rare earths other than lanthanum have been researched and developed from similar viewpoints. In the following explanation, problems in use of lanthanum will be specifically described. Other rare earth elements have similar tendency. Metal lanthanum is easily oxidized during purification. This disadvantageous property makes high purification difficult. Accordingly, there have been no high-purity products. In addition, since metal lanthanum is oxidized and discolored into black within a short time by being left in the air, the handling thereof is difficult. Recently, gate-insulating films in the next-generation MOSFETs are required to reduce the thickness, but the reduction in thickness increases the leakage current due to a tunnel effect in SiO2 used in gate-insulating films, resulting in a difficulty of normal operation.
Consequently, HfO2, ZrO2, A12O3, and La2O3 having high dielectric constants, high thermal stabilities, and high energy barriers against the holes and electrons in silicon have been proposed, as alternatives for SiO2. In particular, La2O3 which is highly valued among these materials, has been examined for its electrical properties, and has been reported as a gate-insulating film in the next-generation MOSFET. However, the subject of research is a La2O3 film, and the properties and behavior of a La element are not reported.
As a method of purifying rare earth metals, a technology of reducing a halide of a rare earth metal using calcium or calcium hydride was proposed about 20 years ago. The examples of the rare earths therein include lanthanum, but the technology itself is merely using a slag separation jig as a means for separating slag, and the problems of metal lanthanum and any method of purification are almost not described (see Japanese Patent Laid-Open No. S63-011628 A).
As described above, the rare earth elements such as lanthanum are paid attention as valuable materials, but high-purity thereof is difficult to be achieved. However, in order to exploit the characteristics of rare earth elements such as lanthanum, it is preferable to reduce the amounts of impurities such as carbon (graphite), Al, Fe, and Cu. Alkali metals, alkaline earth metals, transition metal elements, high melting point metal elements, and radioactive elements affect the characteristics of semiconductors and are also required to be reduced in amounts.
In production of high-purity lanthanum, usually, a starting material of lanthanum fluoride having a purity, excluding the gas components, of 4N or more is reduced with high-purity calcium to prepare lanthanum having a purity of 4N or more, and this lanthanum prepared by reduction is subjected to electron beam melting to remove volatile substances. As a result, high-purity lanthanum having a purity, excluding the gas components, of 4N5 or higher is provided. Rare earth elements other than lanthanum can be highly purified by similar processes. In such cases, however, it is necessary to reduce impurities by using highly purified calcium in the reduction process. Use of a calcium containing a large amount of impurities causes an increase in the amount of impurities contained in the rare earth elements.
As existing technologies, for example, the method disclosed in Japanese Patent Laid-Open No. S58-141349 A includes pre-distillation (3 to 16 hours) at 630° C. to 700° C. to reduce the amount of Mg and main distillation (12 hours) at 900° C. to 920° C. to reduce the amounts of impurities to Mg: 60 ppm (0.006%), Al: 10 ppm (0.001%), Mn: 80 ppm (0.008%), Fe: 10 ppm (0.001%), and Zn<10 ppm (0.001%). However, such a degree of purity of calcium is insufficient. Though the amount of Cu as an impurity is not defined (not described), since water-cooled Cu is necessarily used for handling calcium in a liquid form, a risk of containing a large amount of Cu as an impurity is high.
Japanese Patent Publication No. S63-047780 A discloses a method of producing metal calcium by charging a mixture of such as calcium oxide and aluminum into a retort and performing vacuum distillation, wherein a packed bed of calcium oxide grains is disposed between the mixture and a calcium vapor condenser. However, this method is a technology specialized in reduction of only aluminum in calcium as a reduced agent (Ca) for producing samarium, a refining agent for special steel, and any other impurities in calcium are not described. Thus, the method is not a comprehensive technology for high purification.
Japanese Patent Laid-Open No. H07-076739 A discloses a technology for highly purifying Ca by capturing Mg through a temperature distribution formed in a retort by Al reduction of CaO. This technology is characterized by the structure of the retort that allows purification of calcium to purity of 99.9% or more by one reduction. The analytical values shown are those of Mg only. Thus, the technology is not a comprehensive technology for high purification. As described above, in known technologies, there is no disclosure of technology effective for producing calcium that can achieve comprehensively high purification.